Many optical components integrate photosensitive cells that deliver an electrical signal representative of the received light intensity. Such components are, for example, optical detectors formed from a matrix of photosensitive cells, configured adjacent to one another, whose respective input faces are situated in a common plane. The cells deliver a signal that allows the detected image to be reconstituted from the location of the cells within the matrix. However, since the signal delivered is only representative of the received light intensity, the matrix does not allow discrimination between the various wavelengths detected. In order to reconstitute the colours, a possible method is to use a checkerboard of filters positioned on top of the photosensitive cells, each filter being associated with a particular photosensitive cell.
Filters are chosen to decompose any given light signal into a reduced number of wavelengths. This may, for example, be three primary colours such as blue, green and red. The filters will then be disposed in such a manner that each filter alternates with the other two allowing any given wavelength to be analysed by means of three adjacent photosensitive cells.
The filters generally used are photoresists that cannot withstand temperatures above 300° C. Since the fabrication of the metallization levels requires, amongst other operations, anneal steps at around 400° C., the filters are fabricated after the metallization levels. The filters are therefore situated above the interconnection part of the photosensitive cells, in other words on top of the elements allowing the connection of the sensitive element of the cell to the control circuit and to the data processing circuit.
Conventional connections require four to seven metallization levels. Such metallization levels create a stack that can reach 10 μm in height. In addition, in order to increase the resolution, the width of conventional photosensitive cells is decreasing. Typically, the width of the photosensitive cell may be around 2 μm. Given these dimensions, the aspect ratio (height/width) of such photosensitive cells trend to be much greater than unity, and thus leads to difficulties in fabrication and increases the risks of defects.
For such cells, the filter situated on top of the stack is too far from the sensitive element of the cell, and a light beam of relatively low angle, for example 15°, can cause an overlap of the colors, in other words a light ray passing through a given filter is not detected by the cell associated with the filter but by a neighbouring cell. Moreover, because of the reflexions by the metal of the metallization levels, a light ray passing through a given filter may be detected by a neighbouring cell. Furthermore, the photoresist filters are sensitive to high illumination which reduces their ageing, and most of these filters have also a thickness between 800 nm and 1 μm, required by their optical properties, which increases the height of the stack.
Conventional methods dispose a light guide within the thickness of the interconnection part. The light guide is placed under the filter and collects the light rays that emerge from it and guides the light to the photosensitive element of the associated cell. The guide requires additional fabrication steps which result in high production costs. Moreover, conventional methods fail address or reduce the aspect ratio of the cell.
There is therefore a need for a compact, reliable photosensitive cell with a reasonable cost of fabrication and having an excellent optical performance.